Thin film transistor and method of fabricating the same

ABSTRACT

A thin film transistor including a substrate having first and second regions, a semiconductor layer pattern formed in the first region and the second region, and a first gate insulating layer pattern formed on a channel region of the semiconductor layer pattern of the first region. A second gate insulating layer is formed on the substrate, a first conductive layer pattern is formed above the channel region of the first region and above the semiconductor layer pattern of the second region, and an inter-layer insulating layer is formed on the substrate. A second conductive layer pattern is formed in the first region and above the first conductive layer pattern of the second region. The second conductive layer pattern of the first region is coupled to the semiconductor layer pattern of the first region through the second gate insulating layer and the inter-layer insulating layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2004-0049823, filed Jun. 29, 2004, which is herebyincorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin film transistor (TFT) and methodof fabricating the same and, more particularly, to a TFT and method offabricating the same in which a capacitor's surface area may bedecreased without decreasing its capacity.

2. Discussion of the Background

Generally, a flat panel display device such as an active matrix organiclight emitting display (OLED) has a TFT in each pixel, and each pixelmay be coupled to a gate line, a data line and a power supply line andfurther include a capacitor and an organic light emitting diode. Aplurality of conductive layers may be used to form a gate line, a gateelectrode, a data line, source and drain electrodes, a power supplylayer, an anode electrode, and so forth. Such conductive layers may beelectrically connected to each other by forming a contact hole in aninsulating layer interposed between conductive layers and then buryingthe conductive layer.

FIG. 1 is a plan view showing a conventional active matrix OLED.

Referring to FIG. 1, the conventional active matrix OLED may include aplurality of gate lines 310, a plurality of data lines 320, a pluralityof power supply lines 330, and a plurality of pixels connected to thegate lines 310, the data lines 320, and the power supply lines 330.

Each pixel may include switching and driving TFTs, a capacitor and alight emitting diode. The switching TFT 370 is connected to a gate line310 and a data line 320, the driving TFT 350 drives the light emittingdiode 360 and is connected to the power supply line 330, the capacitor340 maintains a gate-source voltage of the driving TFT 350, and thelight emitting diode 360 emits light to display an image.

The driving TFT 350 has a semiconductor layer 352 including source anddrain regions, a gate electrode 354, and source and drain electrodes 356a and 356 b connected to the source and drain regions through contactholes 355 a and 355 b, respectively. The switching TFT 370 may have thesame structure.

The capacitor 340 includes a bottom electrode 344 connected to a gate ofthe driving TFT 350 and, for example, to the drain electrode of theswitching TFT 370, and a top electrode 346 connected to the power supplyline 330 and, for example, to the source electrode 356 a of the drivingTFT 350. A pixel electrode 361, which may act as an anode of the lightIs emitting diode having an opening 365, may be connected, for example,to the drain electrode 356 b of the driving TFT 350 through a via hole358.

FIG. 2A and FIG. 2B are cross-sectional views showing a sequence offorming a TFT according to a conventional method.

Referring to FIG. 2A, a buffer layer 110 may be formed on the entiresurface of a 20 substrate 100, which is divided into a first region Aand a second region B. The buffer layer 110 may be made of silicon oxidehaving a predetermined thickness, and it may be formed using aplasma-enhanced chemical vapor deposition (PECVD) method. In this case,the buffer layer 110 may prevent impurities from the substrate 100 frompenetrating layers above the buffer layer 110.

Next, an amorphous silicon layer (not shown) having a predeterminedthickness may be deposited on the buffer layer 110. The amorphoussilicon layer may then be crystallized by, for example, excimer laserannealing (ELA), sequential lateral solidification (SLS), metal inducedcrystallization (MIC), metal induced lateral crystallization (MILC), andso forth, and then patterned using photolithography to thereby formpolycrystalline silicon layer patterns 120 in the first region A and thesecond region B within the unit pixel.

A first gate insulating layer 130 may then be formed on the entiresurface of the substrate. In this case, the first gate insulating layer130 may be formed of a silicon oxide (SiO₂) layer or a silicon nitride(SiNx) layer, and it may be about 400 Å to 1,000 Å thick.

A photoresist layer pattern (not shown), which corresponds to a channelregion of the transistor, may then be formed on the first gateinsulating layer 130. Impurity ions may be implanted into thepolycrystalline silicon layer pattern 120 using the photoresist layerpattern as an ion implantation mask, thereby forming source and drainregions 122 in the polycrystalline silicon layer pattern 120 in thefirst region A and a first electrode 124, which may be used as a bottomelectrode of a lower capacitor C1, in the polycrystalline silicon layerpattern 120 in the second region B. The photoresist layer pattern maythen be removed.

Next, a second gate insulating layer 132 may be formed on the first gateinsulating layer 130. The second gate insulating layer 132 may be formedof a SiO₂ layer or a SiNx layer, and it may be about 200 Å to 800 Åthick. Alternatively, the second gate insulating layer 132 may be formedbefore implanting impurity ions into the polycrystalline silicon layerpattern 120 as described above.

A metal layer (not shown) for forming the gate electrode may be formedon the second gate insulating layer 132. The metal layer may be formedof a single layer of molybdenum (Mo) or an alloy such as molybdenumtungsten (MoW), a single layer of aluminum (Al) or an alloy such asaluminum-neodymium (Al—Nd), or a double layer containing these metals.The metal layer may be etched using photolithography to form a gateelectrode 134 in the first region A and a second electrode 136, whichmay be used as a top electrode of the lower capacitor C1, in the secondregion B. In this case, the second electrode 136 may be used as the topelectrode of the lower capacitor C1 and also as a bottom electrode of anupper capacitor C2. Hence, a stacked structure d including the firstgate insulating layer 130 and the second gate insulating layer 132 maybe used as a dielectric layer of the lower capacitor C1.

An inter-layer insulating layer 140 may then be formed to apredetermined thickness on the entire surface of the substrate. In thiscase, a SiO₂ layer and a SiNx layer may be stacked to form theinter-layer insulating layer 140, which may be about 3,000 Å to 5,000 Åthick.

Referring to FIG. 2B, the inter-layer insulating layer 140, the firstgate insulating layer 130, and the second gate insulating layer 132 maythen be etched by photolithography to form contact holes exposing thesource and drain regions 122, respectively.

An electrode material may then be formed on the entire surface of thesubstrate including the contact holes and etched using photolithographyto form source and drain electrodes 150 and 152, which are connected tothe source and drain regions 122, respectively, in the first region Aand a third electrode 154, which may be used as a top electrode of theupper capacitor C2 in the second region B. In this case, a single layerof Mo or an alloy such as MoW, a single layer of Al or an alloy such asAl—Nd, or a double layer containing these metals may be employed for theelectrode material.

A passivation layer 160, which may be made of an inorganic insulatinglayer such as a SiNx layer, may then be formed to a predeterminedthickness on the entire surface of the substrate.

In the method of fabricating the TFT having the above-mentionedstructure, the polycrystalline silicon layer pattern, the gateinsulating layers and the gate electrode are used as the lower capacitorC1, and the gate electrode, the inter-layer insulating layer and thedrain electrode are used as the upper capacitor C2. The lower capacitorC1 and the upper capacitor C2 are formed within the same area. The lowercapacitor C1 has two gate insulating layers as a dielectric layer, andthe upper capacitor C2 has the inter-layer insulating layer as adielectric layer. Hence, a capacitor within a pixel may occupy arelatively large area, and a capacitor having high capacity may berequired as devices are highly integrated. However, the capacitor'ssurface area may need to increase to increase its capacity, whichinevitably causes the aperture ratio of the organic light emittingdisplay to decrease.

SUMMARY OF THE INVENTION

The present invention provides a TFT and method of fabricating the same,in which a surface area of a capacitor may be decreased without changingthe magnitude of capacitance of the capacitor by partially decreasingthe thickness of a gate insulating layer in an organic light emittingdisplay using the dual gate insulating layer to thereby increase anaperture ratio of the organic light emitting display.

Additional features of the invention will be set forth in thedescription which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention.

The present invention discloses a thin film transistor including asubstrate having a first region and a second region, a semiconductorlayer pattern formed in the first region and the second region, and afirst gate insulating layer pattern formed on a channel region of thesemiconductor layer pattern of the first region. A second gateinsulating layer is formed on the substrate, a first conductive layerpattern is formed above the channel region of the first region and abovethe semiconductor layer pattern of the second region, and an inter-layerinsulating layer is formed on the substrate. A second conductive layerpattern is formed in the first region and above the first conductivelayer pattern of the second region. The second conductive layer patternof the first region is coupled to the semiconductor layer pattern of thefirst region through the second gate insulating layer and theinter-layer insulating layer.

The present invention also discloses a method of fabricating a thin filmtransistor including forming a polycrystalline silicon layer pattern ina first region and a second region of a substrate, forming a first gateinsulating layer on the substrate, forming a photoresist layer patternprotecting a channel region of the polycrystalline silicon layer patternof the first region and the second region of the first region, anddoping the polycrystalline silicon layer pattern using the photoresistlayer pattern as a mask to form a source region and a drain region inthe first region and to form a first electrode in the second region,etching the first gate insulating layer using the photoresist layerpattern as a mask to form a first gate insulating layer pattern and thenremoving the photoresist layer pattern, forming a second gate insulatinglayer on the substrate, forming a gate electrode in the first region ofthe substrate and a second electrode in the second region of thesubstrate, forming an inter-layer insulating layer on the substrate,etching the inter-layer insulating layer and the second gate insulatinglayer to expose the source region and the drain region, and forming asource electrode and a drain electrode coupled to the source region andthe drain region, respectively, and forming a third electrode in thesecond region.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a plan view showing a conventional organic light emittingdisplay.

FIG. 2A and FIG. 2B are cross-sectional views showing a sequence offorming a TFT according to a conventional method.

FIG. 3A, FIG. 3B, FIG. 3C, FIG. 3D, FIG. 3E, FIG. 3F and FIG. 3G arecross-sectional views showing a sequence of forming a TFT according toan embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. It will be understood that when an element such as a layer,film, region or substrate is referred to as being “on” another element,it can be directly on the other element or intervening elements may alsobe present. In contrast, when an element is referred to as being“directly on” another element, there are no intervening elementspresent.

FIG. 3A, FIG. 3B, FIG. 3C, FIG. 3D, FIG. 3E, FIG. 3F and FIG. 3G arecross-sectional views showing a sequence of forming a TFT according toan embodiment of the present invention, which may be used to form an Ntype metal oxide semiconductor (NMOS) TFT, a PMOS TFT, or acomplementary MOS (CMOS) TFT.

Referring to FIG. 3A, a buffer layer 210 may be formed on the entiresurface of a substrate 200, which is divided into a first region A and asecond region B. The buffer layer 210 may be made of, for example, asilicon oxide (SiO₂) layer having a predetermined thickness, and it maybe formed using, for example, plasma-enhanced chemical vapor deposition(PECVD). In this case, the buffer layer 210 may prevent impurities fromthe substrate 200 from penetrating layers above the buffer layer 210.

Next, an amorphous silicon layer (not shown) having a predeterminedthickness may be deposited on the buffer layer 210. The amorphoussilicon layer may then be crystallized using, for example, excimer laserannealing (ELA), sequential lateral solidification (SLS), metal inducedcrystallization (MIC), metal induced lateral crystallization (MILC), andso forth. The polycrystalline silicon layer may then be patterned using,for example, photolithography to thereby form polycrystalline siliconlayer patterns 220 a and 220 b in first region A and second region B,respectively, within the unit pixel.

A first gate insulating layer 230 may then be formed on the entiresurface of the substrate. In this case, the first gate insulating layer230 may be formed of, for example, a SiO₂ layer or a silicon nitride(SiNx) layer, and it may be about 400 Å to 1,000 Å thick, preferablyabout 800 Å thick.

Referring to FIG. 3B, a photoresist layer pattern 238, which correspondsto a channel region of the transistor, may then be formed on the firstgate insulating layer 230 in the first region A. Impurity ions may beimplanted into the polycrystalline silicon layer patterns 220 a and 220b using the photoresist layer pattern 238 as an ion implantation mask,thereby forming source and drain regions 222 a and a first electrode 222b, which may be used as a bottom electrode of a lower capacitor C1.Here, the ion implantation process may be performed using an n+ impurityor a p+ impurity as a dopant. When the TFT is to be a CMOS TFT, the n+impurity may be implanted into the first electrode 222 b.

Referring to FIG. 3C, the first gate insulating layer 230 may be etchedusing the photoresist layer pattern 238 as an etch mask to form a firstgate insulating layer pattern 231 above the channel region of thetransistor.

The photoresist layer pattern 238 may then be removed.

Alternatively, in the case of the NMOS TFT having a lightly doped drain(LDD) region, the first gate insulating layer pattern 231 may extend tothe LDD region beyond the channel region.

Referring to FIG. 3D, a second gate insulating layer 232 may be formedon the entire surface of the substrate. The second gate insulating layer232 may be formed of, for example, SiO₂ or SiNx, and it may be about 200Å to 800 Å thick, and is preferably formed of SiNx at a thickness ofabout 400 Å.

Referring to FIG. 3E, a first conductive layer (not shown) may be formedon the second gate insulating layer 232. The first conductive layer maybe formed of, for example, a single layer of Mo or an alloy such as MoW,a single layer of Al or an alloy such as Al—Nd, or a double layercontaining these metals. The first conductive layer may be etched using,for example, photolithography to form a first conductive layer patternincluding a gate electrode 234 formed in the first region A and a secondelectrode 236 formed in the second region B. In this case, the secondelectrode 236 may be used as the top electrode of the lower capacitor C1and also as a bottom electrode of an upper capacitor C2. The first gateinsulating layer pattern 231 and the second gate insulating layer 232may be used as a gate insulating layer d′, and they may be about 600 Åto 1,800 Å thick in the first region A. Additionally, the second gateinsulating layer 232 may be used as a dielectric layer d″ of the lowercapacitor C1, and it may be about 200 Å to 800 Å thick, preferably about400 Å thick, in the second region B.

Referring to FIG. 3F, an inter-layer insulating layer 240 may then beformed to a predetermined thickness on the entire surface of thesubstrate. The inter-layer insulating layer 240 may be formed of, forexample, a SiO2 layer, and it may be about 3,000 Å to 5,000 Å thick,preferably about 4,000 Å thick.

Referring to FIG. 3G, the inter-layer insulating layer 240 and thesecond gate insulating layer 232 may be etched using, for example,photolithography to form contact holes exposing the source and drainregions 222 a, respectively.

A second conductive layer may then be formed on the entire surface ofthe substrate including the contact holes and etched using, for example,photolithography to form a second conductive layer pattern includingsource and drain electrodes 250 and 252 formed in the first region A anda third electrode 254 formed in the second region B. The source anddrain electrodes 250 and 252 may be coupled to the source and drainregions 222 a, respectively, and the third electrode 254 may be used asa top electrode of the upper capacitor C2. Here, for example, the secondconducive layer may be made of a single layer of Mo or an alloy such asMoW, a single layer of Al or an alloy such as Al—Nd, or a double layercontaining these metals.

A passivation layer 260, which may be formed of, for example, aninorganic insulating layer such as a SiNX layer having a predeterminedthickness, may then be formed on the entire surface of the substrate.

The capacitor is vertically formed within the same area in which thelower and upper capacitors C1 and C2 are formed as shown in FIG. 3G.

For example, when the first gate insulating layer 230 is about 800 Åthick, the second gate insulating layer 232 is about 400 Å thick, andthe inter-layer insulating layer 240 is about 4,000 Å thick, the surfacearea of the capacitor according to an embodiment of the presentinvention may be expressed by the following equation 1: $\begin{matrix}{A = \frac{Cst}{\frac{ɛ\quad{ILD}}{dILD} + \frac{1}{\frac{dGI1}{ɛ\quad{GI1}} + \frac{dGI2}{ɛ\quad{GI2}}}}} & \left\lbrack {{Equation}\quad 1} \right\rbrack\end{matrix}$

(ε is a dielectric constant, d is a thickness of the dielectric layer,Cst is a capacitance, ILD is an inter-layer insulating layer, GI1 is afirst gate insulating layer, and GI2 is a second gate insulating layer)

and where the lower capacitor C1 has only the second gate insulatinglayer 232 as the dielectric layer (i.e. GI1=0), the surface area of thecapacitor may be expressed by the following equation 2: $\begin{matrix}{A^{\prime} = \frac{Cst}{\frac{ɛ\quad{ILD}}{dILD} + \frac{ɛ\quad{GI2}}{dGI2}}} & \left\lbrack {{Equation}\quad 2} \right\rbrack\end{matrix}$

When a pixel includes a five transistor and two capacitor structureunder the conditions described above, the surface area of the capacitormay decrease by about 27% and the pixel's resultant aperture ratio maybe enhanced by 10% or more.

Additionally, when a pixel includes a two transistor and one capacitorstructure, the surface area of the capacitor may decrease by about 27%and the pixel's resultant aperture ratio may be enhanced by 2.7% ormore.

According to embodiments of the present invention, when the OLED using adouble gate insulating layer is fabricated, the thickness of the gateinsulating layer in the TFT region may be different from that in thecapacitor region. Accordingly, the electrical characteristics of thegate electrode may be maintained and the surface area of the capacitormay decrease without changing the capacitance of the capacitor.Decreasing the surface area of the capacitor may enhance the apertureratio of the OLED, and the effect of the enhanced aperture ratio may bemore enhanced when the capacitance is increased to decrease the leakagecurrent.

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A thin film transistor, comprising: a substrate having a first regionand a second region; a semiconductor layer pattern formed in the firstregion and the second region; a first gate insulating layer patternformed on a channel region of the semiconductor layer pattern of thefirst region; a second gate insulating layer formed on the substrate; afirst conductive layer pattern formed above the channel region of thefirst region and above the semiconductor layer pattern of the secondregion; an inter-layer insulating layer formed on the substrate; and asecond conductive layer pattern formed in the first region and above thefirst conductive layer pattern of the second region, wherein the secondconductive layer pattern of the first region is coupled to thesemiconductor layer pattern of the first region through the second gateinsulating layer and the inter-layer insulating layer.
 2. The thin filmtransistor of claim 1, wherein the semiconductor layer pattern is apolycrystalline silicon layer pattern.
 3. The thin film transistor ofclaim 1, wherein the semiconductor layer pattern of the first region isa channel region, a source region and a drain region, and thesemiconductor layer pattern of the second region is a bottom electrodeof a lower capacitor.
 4. The thin film transistor of claim 1, whereinthe first gate insulating layer pattern is formed of any one of asilicon oxide layer and a silicon nitride layer.
 5. The thin filmtransistor of claim 4, wherein the first gate insulating layer patternis about 400 Å to about 1,000 Å thick.
 6. The thin film transistor ofclaim 1, wherein the second gate insulating layer is formed of any oneof a silicon oxide layer and a silicon nitride layer.
 7. The thin filmtransistor of claim 6, wherein the second gate insulating layer is about200 Å to about 800 Å thick.
 8. The thin film transistor of claim 1,wherein the first conductive layer pattern of the first region is a gateelectrode, and the first conductive layer pattern of the second regionis a top electrode of a lower capacitor and a bottom electrode of anupper capacitor.
 9. The thin film transistor of claim 1, wherein thesecond conductive layer pattern of the first region is a sourceelectrode and a drain electrode, and the second conductive layer patternof the second region is a top electrode of an upper capacitor.
 10. Amethod of fabricating a thin film transistor, comprising: forming apolycrystalline silicon layer pattern in a first region and a secondregion of a substrate; forming a first gate insulating layer on thesubstrate; forming a photoresist layer pattern protecting a channelregion of the polycrystalline silicon layer pattern of the first region;doping the polycrystalline silicon layer pattern of the first region andthe second region using the photoresist layer pattern as a mask to forma source region and a drain region in the first region and to form afirst electrode in the second region; etching the first gate insulatinglayer using the photoresist layer pattern as a mask to form a first gateinsulating layer pattern and then removing the photoresist layerpattern; forming a second gate insulating layer on the substrate;forming a gate electrode in the first region of the substrate, and asecond electrode in the second region of the substrate; forming aninter-layer insulating layer on the substrate; etching the inter-layerinsulating layer and the second gate insulating layer to expose thesource region and the drain region; and forming a source electrode and adrain electrode coupled to the source region and the drain region,respectively, and forming a third electrode in the second region. 11.The method of claim 10, wherein the first electrode is a bottomelectrode of a lower capacitor.
 12. The method of claim 10, wherein thefirst gate insulating layer is formed of any one of a silicon oxidelayer and a silicon nitride layer.
 13. The method of claim 12, whereinthe first gate insulating layer is about 400 Å to about 1,000 Å thick.14. The method of claim 10, wherein the second gate insulating layer isformed of any one of a silicon oxide layer and a silicon nitride layer.15. The method of claim 14, wherein the second gate insulating layer isabout 200 Å to about 800 Å thick.
 16. The method of claim 10, whereinthe second electrode is a top electrode of a lower capacitor and abottom electrode of an upper capacitor.
 17. The method of claim 10,wherein the third electrode is a top electrode of an upper capacitor.18. A flat panel display device comprising the thin film transistor ofclaim
 1. 19. A thin film transistor, comprising: a substrate having afirst region and a second region; a semiconductor layer pattern formedin the first region and the second region; a first conductive layerpattern formed above a channel region of the semiconductor layer patternof the first region and above the semiconductor layer pattern of thesecond region; a second conductive layer pattern formed in the firstregion and above the first conductive layer pattern of the secondregion, wherein a first gap between the first conductive layer patternand the semiconductor layer pattern of the first region is greater thana second gap between the first conductive layer pattern and thesemiconductor layer pattern of the second region.
 20. The thin filmtransistor of claim 19, wherein the first gap is formed by a first gateinsulating layer pattern formed on the channel region and a second gateinsulating layer formed on the substrate, and the second gap is formedby the second gate insulating layer.